Efforts are being made in miniaturization and integration of semiconductor devices to meet the demand for electronic devices of miniature sizes in recent years. Thus, the semiconductor device of the WCSP structure, i.e. a semiconductor device whose outer shape is made as close to a semiconductor chip as possible to reduce the size of the semiconductor device has been proposed.
The semiconductor device of the WCSP structure needs densely arranged output terminals for connection with the outside. So, column-shaped terminals electrically connected to connection pads formed on a chip surface of the semiconductor device and projecting vertically from the chip surface are used as such output terminals. Generally, such column-shaped terminals are referred to as column electrodes, projecting electrodes, or bump electrodes.
When the semiconductor device of the WCSP structure is mounted on a circuit board, the tips of the columnar electrodes are soldered to connection pads provided on the surface of the circuit board, and after that, the gap between the semiconductor device and the circuit board is filled with a sealing resin.
Since it is difficult and takes much time to fill the narrow gap with the sealing resin uniformly, it has been proposed to form a resin film in advance on the chip surface of the semiconductor device to such a thickness that the tips of the column electrodes are exposed at the surface of the resin film, as disclosed in Japanese Laid-Open Patent No. 10-79362, for example.
In this case, it is preferable to form the sealing film from a resin of high elasticity (hard resin) for increasing mechanical strength of the columnar electrode section of the semiconductor device. However, for absorbing distortion occurring at the solder joint section because of the difference in thermal expansion coefficients between the semiconductor chip and the circuit board and preventing cracks from occurring at the solder joint section, it is preferable to form the sealing film from a resin of low elasticity (soft resin). To increase the reliability of the semiconductor device, it is necessary to achieve both of them. So, a double-layer structure in which the sealing resin film is comprised of a soft resin layer and a hard resin layer is proposed in the Japanese Laid-Open Patent No. 10-79362.
However, employing such a double-layer structure does not necessary provide a reliable semiconductor device, since the effects of the strength improvement of the columnar electrode section and the distortion absorption vary with the thicknesses of the hard resin layer and the soft resin layer as well as the ratios of them to the total thickness. In addition, although the Japanese Laid-Open Patent No. 10-79362 suggests forming the double-layer structure by a compression moulding process or a transfer moulding process, it is very difficult to form such a double-layer structure by such processes. To be more specific, technically, it is very difficult to form even the first one of the two resin layers by any of such moulding processes, and still more difficult to form a stack of the two resin layers by one resin-filling process.
The present invention has been made with an object of providing a reliable semiconductor device in which the columnar electrode section has sufficient strength, and cracks due to the difference in thermal expansion coefficients between the semiconductor chip and the circuit board are prevented from occurring. Another object of the present invention is to provide such a semiconductor device at low cost.